For practical operation of elevator systems, it is important that the elevator car should stop at the desired position at a floor. In other words, the stopping accuracy of the elevator car has to be within a certain tolerance. It is clear that if the floor of the elevator car remains e.g. 15 cm above the floor level, there is something wrong with the control of stopping.
In new elevator systems, the elevator control system generally comprises an integrated location system. This allows the stopping accuracy of the elevator car to be monitored and, if necessary, corrected on the basis of accumulated stopping accuracy data. However, not all elevator systems have an integrated system for monitoring the stopping accuracy of the elevator.
Based on the monitoring of stopping accuracy, it is possible to control e.g. the condition of the brakes used to decelerate the elevator car and the operation of the car load weighing device. Defective operation of the brakes naturally results in an inaccuracy of stopping of the elevator car.
In prior art, the stopping accuracy of an elevator car has also been determined using e.g. a magnetic zone. A magnetic zone is a zone of a few centimeters, within which the elevator car should stop in a normal situation. A measurement utilizing a magnetic zone only indicates whether the elevator car stopped within that zone or not. Therefore, magnetic zone measurement does not give any precise information regarding stopping accuracy. In other methods of measuring stopping accuracy, e.g. various detectors are used to indicate the position where the elevator car stops. A problem with the use of detectors is that they are very difficult to mount at a precise position. If the detectors are not mounted at exactly the right positions, then the measurement of stopping accuracy of the elevator car is no longer accurate.
Naturally, to allow measurement of the stopping accuracy of an elevator car, solutions capable of accurate measurement of the stopping accuracy of the elevator car can be installed in the elevator car, in the elevator shaft and/or in the machine room. However, such solutions are expensive, and they are not reasonable for mass production in respect of their price/quality ratio.
It is possible to calculate the position of the elevator car e.g. from acceleration data by first integrating acceleration as velocity and then velocity as position. The problem is that integration is very sensitive to offset-type errors because an error will accumulate over the entire integration cycle. Especially in double integration, the standard error increases quadratically
                              ∫                      ∫                                          a                0                            ⁢                              ⅆ                                  t                  2                                                                    =                              1            2                    ⁢                      a            0                    ⁢                      t            2                                              (        1        )            where a0 is the offset term of acceleration measurement. An acceleration sensor can never be mounted in a completely straight position, and besides, due to the car load, the acceleration sensor is always somewhat askew. In addition, electrical resetting of the transducer-amplifier-A/D converter of the chain is never completely free of errors. Due to the above-mentioned reasons, vertical acceleration measurement of the car always contains a constant term a0=am+ae+n, where am is a constant error caused by mechanical factors, ae is the reset error of the electric chain and n is the measurement noise. The constant term a0 accumulates into position measurement according to equation (1). The average measurement noise is zero and its effect disappears in the integration process. The constant term arising from the tilt error isam=(1−cos α)·g  (2)where α is the tilt angle from the horizontal plane and g is the acceleration 9.81 m/s2 of the Earth. If the elevator takes e.g. 4.5 s to travel between successive floors (elevator speed 1 m/s, acceleration 0.8 m/s2, distance between floors 3.2 m), then according to equations (1) and (2) e.g. a 2.5-degree tilt error results in an error of about 10 cm in the position integrated from the acceleration measurement. This accuracy is not sufficient for the monitoring of stopping accuracy.
In existing elevators with no accurate location system, there is no sufficiently accurate system for monitoring the stopping accuracy of the elevators. In the course of decennia, there have been tens if not hundreds of elevator manufacturers and consequently even a greater number of different models. For this reason, a most diverse variety of electric and mechanical implementations are found in elevators.
Reference was made above to a stopping window implemented as a magnetic zone, within which the elevator should stop. The tolerance of the stopping window is adjusted mechanically during installation, and the width of the window depends on the implementation of the elevator drive. In simple implementations where it is known that the elevators have poor stopping characteristics, the stopping window is made wide. In the case of the most modern drives, which employ inverters and speed measurement and in which the stopping accuracy should be better by nature, the window is set to a narrower width.
Mechanical basic adjustment and subsequent adjustment/modification of stopping windows is a time-consuming and difficult task. In addition, in present condition monitoring systems, part of the system is typically placed on the top of the car (stopping accuracy data) while some of the signals are obtained from the elevator panel (start command on/off, to indicate whether the elevator is moving). However, a distributed implementation involves problems:                connection to the elevator control panel and finding the correct signals in it and connecting to them, and        for data transfer between the devices in the machine room and on the car top, an extra car cable has to be installed.        
Based on the circumstances described above, there are considerable drawbacks in present-day condition monitoring systems in existing elevators, especially in respect of measurement and monitoring of stopping accuracy.